The present invention relates to radar systems and, in particular, to systems having a communication capability added to its designed radar capability.
Systems of the type under consideration are used for a number of purposes one of which is to track semi-active radar guided missiles or the like and, as needed, to transmit course-correction commands or the like over a data link. By way of illustration, such a system may utilize pulse doppler radar and, to achieve the desired data communication capability, some type of carrier frequency modulation is employed. However, since the carrier frequency itself is modulated the radar function must be interrupted during each message transmission. In other words, the radar transmitter must be time-shared between the radar and the data link functions. Similar situations exist in comparable systems such as those used for secure communication purposes or for IFF interogatoins. For example, secure communication radars may employ a highly directional transmitter antenna for message purposes but, again the transmitter must be time-shared rather than being continuously available for uninterrupted operation in either mode.
According to the present invention, a radar compatible data link system is provided by utilizing pulse repetition modulation (PRF) for the data link function. More specifically, the transmitter section of the system generates and transmits a radar carrier wave the transmitted frequency of which remains intact or unmodulated to permit the radar function to continue unaffected by the data transmission. As indicated, PRF modulation provides the data link capability and, for this purpose, the system utilizes a plurality of PRF generators of differing repetition rates. The output of theses generators are selectively applied to pulse modulate the carrier wave. A digital input, in turn, controls the selection of the PRF generators to the extent that, for example, a binary ‘1’ will select one of the generators, while a binary ‘0’ will select another. The transmitted wave thus has the unmodulated carrier frequency to provide the radar function. It also contains the message data in the PRF sideband components of the wave. A command receiver selectively processes the sideband information to reconstruct the digital data.